Plantar pressure and shear stress reduction insole for diabetic foot ulceration

ABSTRACT

The main purpose of this invention is to develop a new insole design which can significantly reduce the plantar pressure under the forefoot and heel, and can be used as replacement insole for the diabetic shoe, diabetic healing shoe, and removable cast walker.  
     This new insole is consisting of shock absorbing and shear reducing composite layers of Poron or soft EVA for the bottom layer, Plastazote or cushioned polymer gel for the middle layer, and a closed cell Neoprene top cover. These materials are commercially available in sheets.  
     The main advantage and uniqueness of this invention, comparing to other existing insoles, are the evenly spaced holes throughout the insole. The holes will significantly reduce the direct plantar pressure and shear stress dynamically exerting on the plantar skin upon loading. When there is a focal point of pressure, the holes will be distorted or stretched to the direction of the pressure which will also allow the insole material to distorted or “give” resulting in reduction of the peak plantar pressure and the associated shear stress. This will also eliminate any pressure transferring problems as encountered in other insoles. Removing the pressure will allow the insole material to return back to the original state. Therefore, dynamic direct plantar pressure and shear stress can be significantly reduced by these holes.

BACKGROUND OF THE INVENTION

[0001] There are 16 million people or 5.9% of the population in theUnited States that have diabetes. Many people first become aware thatthey have diabetes when they develop one of its major complications suchas blindness, heart disease, stroke, peripheral vascular disease, andnumbness or neuropathy in the feet. High blood sugar level also affectsthe body immune system and cause delayed wound healing.

[0002] Diabetic foot complications are the most common cause ofnon-traumatic lower extremity amputations in the United States. The riskof lower extremity amputation is 15 to 46 times higher in diabetics thanin normal persons, and the majority of diabetic foot complications beginwith the formation of skin ulcers on the bottom of the foot.

[0003] One of the main causes for diabetic ulceration is the increase inplantar pressure on the bottom of the foot; especially, the forefoot andthe heel area. Foot deformities, which are common in diabetic patients,lead to focal area of high pressure. When an abnormal focus of pressureis coupled with lack of sensation, a foot ulcer can develop. Therefore,off-loading plantar foot pressure is an important component in treatingdiabetic foot ulcerations.

DESCRIPTION OF PRIOR ART

[0004] There are many off-loading techniques and devices available, eachhaving specific applications according to the anatomic location of thewound. Off-loading devices made for non-ambulatory use include airflowmattresses, soft padding for the bed and wheelchair, and heel protectorssuch as the multipodus boot that suspends the limb to completely removepressure from the problem area.

[0005] For ambulatory patients with plantar foot ulcerations, theultimate off-loading device is a total contact cast, which acts totransfer weight away from the foot and redistributes the forces ofweight bearing proximally onto the leg. A removable cast walker deviceperforms much of the same off-loading as a total contact cast but isoften more easily tolerated by the patient because it is removable forbathing and for daily wound care. Other off-loading but less idealdevices include a half-shoe (or so-called “wedge shoe”), apostoperative/surgical shoe with soft accommodative padding layer. Arandomized clinical study was conducted to compare the effectiveness oftotal-contact casts, removable cast walkers, and half-shoes to healneuropathic foot ulcerations in individuals with diabetes. The studyreports that a significantly higher proportion of patients were healedin 12 weeks in the total contact cast group when compared with the twoother modalities (89.5% vs. 65%, and 58.3%) (Off-loading the diabeticfoot wound: a randomized clinical trial, Diabetes Care 2001November;24(11):2016)

[0006] U.S. Pat. No. 5,197,942 (Brady) describes wearable foot orthoseswith wound aperture to offload plantar pressure when a patient walk.However, most diabetic patients with foot ulceration have insensate footand orthoses with hard material could create additional plantar footulceration.

[0007] U.S. Pat. No. 5,329,705 (Grim) describes removing grids ofremovable resilient hexagon elements in the insole to offload theplantar foot pressure. However, this practice will subsequentlytransferring peak plantar pressure to other part of the foot.Furthermore, as the pressure is reduced through the aperture (fromremoving the hexagon elements) it will put additional pressure aroundthe wound edge and subsequently creating more skin undermining andcalluses around the wound.

[0008] U.S. Pat. No. 5,483,757 (Frykberg) describes insole withpreferably multiple evenly spaced holes extends through for breathing.These are limited number of small holes with the main purpose for aircirculation through the insole. These hole design has no effective peakplantar pressure reduction.

[0009] U.S. Pat No. 5,797,862, and U.S. Pat. No. 6,083,185 (Lamont)describes diabetic boot and insole consisting of an upper layerplastazote and lower layer of poron material which laminated togetherwith additional metatarsal and scaphoid paddings to offload footpressure. This kind of material is commercially available in sheetswhich can be cut out to fit into a shoe or boot. However, it still doesnot have the offloading capabilities such as the new invention (seetesting below).

SUMMARY OF THE INVENTION

[0010] The main purpose of this invention is to develop a new insoledesign which can significantly reduce the plantar pressure under theforefoot and heel, and can be used as replacement insole for thediabetic shoe, diabetic healing shoe, and removable cast walker.

[0011] This new insole is consisting of shock absorbing and shearreducing composite layers of Poron or soft EVA for the bottom layer,Plastazote or cushioned polymer gel for the middle layer, and a closedcell Neoprene top cover. These materials are commercially available insheets.

[0012] The main advantage and uniqueness of this invention, comparing toother existing insoles, are the evenly spaced holes throughout theinsole. The holes will significantly reduce the direct plantar pressureand shear stress dynamically exerting on the plantar skin upon loading.When there is a focal point of pressure, the holes will be distorted orstretched to the direction of the pressure which will also allow theinsole material to distorted or “give” resulting in reduction of thepeak plantar pressure and the associated shear stress. This will alsoeliminate any pressure transferring problems as encountered in otherinsoles. Removing the pressure will allow the insole material to returnback to the original state.

[0013] Therefore, dynamic direct plantar pressure and shear stress canbe significantly reduced by these holes.

[0014] This insole can also be used to offload plantar pressure ofdeformed foot which has bony prominences due plantar fat pads atrophy inpeople with rheumatoid arthritis.

[0015] This new insole, when used in conjunction with a shoe or withremovable cast walker, will replace the total contact cast, which is theultimate off-loading device for treating diabetic foot ulcerations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1. is top view of the right insole

[0017]FIG. 2. is a side elevation view of the right insole

[0018]FIG. 3. is a cross sectional view of the right insole, taken alongline A-A of FIG. 1

[0019]FIG. 4. is top view of the right insole, of interest area “A”,showing non-loaded insole

[0020]FIG. 5. is a top view of the right insole, of interest area “A”,showing a loaded insole with focal pressure point

[0021]FIG. 6. is a cross sectional view of the region of interest “A”upon loading, taken along line B-B

DETAILED DESCRIPTION

[0022] This invention of the off-loading insole for diabetic ulcerationconsists of a shock absorbing composite layers insole, which also hasadditional capability to reduce the peak plantar foot pressure and shearstress on the plantar skin by evenly spaced holes through the insole.See FIGS. 1, 2, 3.

[0023] The insole main shape can be of a foot, existing shoe insole, orthe outline of a post surgical shoe or removable cast walker.

[0024] The bottom layer material is soft EVA with a thickness of ¼″ to½″ which has excellent shock absorption, high direct impact performance,and high resiliency.

[0025] The middle layer material can be either pink Plastazote, orcushioned polymer gel with a thickness of ¼″ to ⅜″ to mainly absorb thedirect plantar pressure and to reduce the shearing stress.

[0026] The top layer material is closed cell Neoprene with a thicknessof ⅛″ and is used as the top cover for the insole.

[0027] All three materials are glued together to form a compositelayered insole which is ½″ to 1″ thick. These materials are commerciallyavailable. See FIG. 2.

[0028] Main shear stress reduction is accomplished by the middle layersince it has the elastic property to “slide” or “give” between the topand bottom layers when the direction of force is parallel or nearparallel to the insole surface.

[0029] Additional plantar pressure and shear stress reduction areaccomplished by evenly spaced holes drilled or punched through alllayers of the insole. These holes, can be varied in sizes and shapes toaccommodate different weight loading requirements, and are separated byat least ¼″ to 1″ away from each others.

[0030]FIG. 4 shows shape of the holes when in a relaxed or non-loadedposition.

[0031]FIGS. 5 and 6 show that when there is a focal point of pressure,the holes will be distorted or stretched to the direction of thepressure which will also allow the insole material to distorted or“give” resulting in reduction of the peak plantar pressure and theassociated shear stress. Removing the pressure will allow the insolematerial to return back to the original state. Therefore, additionaldynamic direct plantar pressure and shear stress can be significantlyreduced by these holes.

Test Results of the Peak Plantar Pressure of Various Insoles

[0032] We conducted a study to determine the plantar pressure under themedial forefoot and under the heel using:

[0033] 1. Plain surgical shoe

[0034] 2. Laminated insole with ¼″ pink Plastazote and ⅛″ PPT. This isthe normal insole material for diabetic shoe.

[0035] 3. Laminated insole with ¼″ soft EVA for the bottom layer, ¼″pink Plastazote for the middle layer, and ⅛″ Spenco for the top layer.One has no hole and another one has {fraction (3/16)}″ holes drilledthrough all layers of the insole and are evenly spaced at ½″.

[0036] The plantar pressures were measured using the F-Scan in-shoepressure measurement system (TekScan, South Boston, Mass.). The test wasperformed on a treadmill at a walking speed of 1.5 mph with a femalesubject weights 135 lbs and wearing a surgical shoe with and without theinsoles above. Data were collected and tabulated as follow: PlainSurgical Shoe Pressure Pressure Under Medial Forefoot 15 psi (10.35N/Cm2) Pressure Under Heel 20 psi (13.8 N/Cm2)

[0037] Insole with PPT and pink Plastazote and surgical shoe Number 1Insole Without Holes Pressure Under Medial Forefoot 14 psi (9.66 N/Cm2)Pressure Under Heel 18 psi (12.42 N/Cm2)

[0038] Insole with ¼″ soft EVA for the bottom layer, ¼″ pink Plastazotefor the middle layer, and ⅛″ Spenco for the top layer, and surgical shoeNumber 2 insole Without Holes With Holes Pressure Under Medial 13 psi(8.98 N/Cm2) 10 psi (6.9 N/Cm2) Forefoot Pressure Under Heel 17 psi(11.73 N/Cm2) 15 psi (10.35 N/Cm2)

[0039] As indicated in the tables above, the peak plantar pressure wasfurther reduced by using insoles with evenly spaced holes.

[0040] Similar study conducted by Lavery LA and cohorts in “ReducingDynamic Foot Pressures in High-risk Diabetic Subjects With FootUlcerations” (Diabetes Care 19(8):818-821, 1996) reports mean peakpressure for ulcers under the 1^(st) metatarsal heads (medial forefoot)for the Total Contact Cast, DH Pressure Relief Walker (Royce Medical,Camarillo, Calif.), Aircast Pneumatic Diabetic Walker (Aircast, Summit,N.J.), Extra Depth Shoe as 7N/Cm2, 8N/Cm2, 12.3N/Cm2, and 39.5N/Cm2respectively.

[0041] Another study conducted by Armstrong D G and cohorts in “TotalContact Casts and Removable Cast Walkers” (J Am Podiatric MedicalAssociation 89(1):50-53, 1999) reports peak plantar heel pressure forthe Total Contact Cast, DH Pressure Relief Walker Aircast PneumaticDiabetic Walker, and P W Minor Extra Depth Shoe (PW Minor and Son,Batavia, N.Y.) as 18N/Cm2, 19N/Cm2, 20N/Cm2, and 25N/Cm2 respectively.

[0042] Comparing to the above studies for peak plantar pressure underthe medial forefoot and the plantar heel, there is not a substantialdifference between the new invention insole and the Total Contact Cast,which is a gold standard for offloading diabetic foot ulceration, tooffload the medial forefoot (6.9N/Cm2 vs 7N/Cm2). However, the newinsole is much better to offload the plantar heel pressure (10.35N/Cm2vs 18N/Cm2).

1. What I claim as my invention is for evenly spaced holes drilled orpunched through/partially through all any composite layers andthroughout the diabetic healing shoe insole, or removable cast walkerinsole, to reduce peak plantar foot pressure and shear stress. The shapeof the holes can be round, square, oval, hexagon, octagon, or can have asmaller round/octagon diameter on one surface and a larger round/octagondiameter on the opposite surface. These holes, can be varied in sizesfrom {fraction (1/16)}″ to ½″ to accommodate different weight loadingrequirements, and are separated by at least ¼″ to 1″ away from eachothers and in a square, round, hexagon, octagon, or alternatingpatterns. When there is a focal point of pressure, the holes will bedistorted or stretched to the direction of the pressure which will alsoallow the insole material to distorted or “give” resulting in reductionof the peak plantar pressure and the associated shear stress. This willalso eliminate any pressure transferring problems as encountered inother insoles. Removing the pressure will allow the insole material toreturn back to the original state: